So you got yourself a WaterRower, but you're looking for something more to enhance your journey?

Here are two options you can get and what they offer:

SmartRow (Android, iOS)

It's a smart pulley combined with a mobile app (iOS & Android). It provides a more accurate way to measure your applied power & other metrics than the S4 monitor.
It has some workouts (custom workouts coming soon, iOS only for now), async multiplayer, and a wealth of data that it can also push to Strava.

Costs: The pulley costs around $250-$300 flat and it's free - no need for a monthly subscription.
Links: [Official site] [/r/SmartRow] [Strava SmartRow group]

Ergatta

It's a tablet with an arm holder that's installed on the WaterRower and connects to the S4 monitor.
You can get a WaterRower integrated with Ergatta or an upgrade kit if you already have a WaterRower.
Ergatta offers a huge library of workouts (new ones added every week) that guide you in real-time on stroke rate, speed or both. It also has multiplayer races where you compete with other users, and monthly challenges for achievements. The software/platform is being actively developed with new features.

Costs: Ergatta-integrated WaterRower is around $2200, and the upgrade kit if you already have a WaterRower is around $550. (check website for accurate up-to-date prices and extra shipping/installation fees + taxes). Also, to use their platform there's a $29/mo ($290/yr) subscription.
Links: [Official site] [/r/ErgattaRower] [Strava Ergatta group]

Coxswain (Android)

Free open-source Android app that connects to the S4 monitor, either (i) via USB-OTG to your Android device, or (ii) via Bluetooth that requires the Bluetooth ComModule (S4 comms 36 is not supported) .
Record your workout while watching YouTube videos or other media. Row against previous workouts. Exports workouts to TCX, Google Fit.

Costs: Free, just needs an Android device, USB-OTG cable($4) or Bluetooth ComModule (US$60).
Links: [Official site] [Github source]

We-Row (Windows, Mac, Linux)

Laptop/desktop application that connects to S4 monitor via USB.
Connect and race against other WaterRower owners. Set up your own private training session. Ability to store your data and access any time and also share with other users.

Costs: Free, just needs S4 monitor (2.10 Firmware).
Links: [Official site] [Alternative official? site]

EXR Game (BETA) (Windows, Android/iOS)

The Zwift of indoor rowing. Virtual world where you control your avatar with your WaterRower. Laptop/desktop/mobile application that connects to the S4 monitor via Bluetooth (ComModule).
More info at r/waterrower/comments/lw4nj8/exr_game_adds_support_for_waterrower/

Costs: 1yr post-launch membership for $49.99 limited-time discount, $99.99/yr after launch.
Requires: S4 monitor with Bluetooth ComModule
Links: [Official site]

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For Tech Tinkerers

PiRowFlo (Android, iOS)

Powerful WaterRower data flow magic proxy. Connect your S4 monitor or SmartRow to a Raspberry Pi and let it transmit them to (Android/iOS +?) apps through Bluetooth or ANT+. See more detailed features at https://github.com/inonoob/pirowflo#features.

Costs: Free & open-source.
Requires: S4 monitor w/ USB, Raspberry Pi, and depending on your case some USB dongles (BT/ANT+).
Links: [GitHub]

-- Feel free to post or PM me any similar offerings for WaterRowers.
Note: the information here might get outdated, always check the official sites.